A recent innovation of Texas Instruments Incorporated of Dallas Texas, is the digital micromirror device (DMD). The DMD is suitable for use in video displays, projectors and hard copy xerographic printers. The DMD is a monolithic single-chip integrated circuit spatial light modulator (SLM), comprised of a high density array of 17 micron square movable micromirrors. These micromirrors are fabricated over an address circuitry including an array of SRAM cells and address electrodes. Each mirror forms one pixel of the DMD array, and is bistable, that is to say, deflectable to one of two stable positions. A source of light directed upon and incident to the mirror array will be reflected in one of two directions as a function of the mirror position. In one stable "on" mirror position, incident light will be reflected to a collector lens and focused on a display screen in the case of a display, or to an exposure module, such as a photoreceptor drum, in the case of a hard copy printer. In the other "off" mirror position, incident light directed on the mirror will be deflected to a light absorber. Each mirror of the array is individually controlled to form a light image, this image being directed into the collector lens. For a display, the collector lens and a light prism ultimately focus and magnify the light image from the pixel mirrors onto a display screen and produce a viewable image. If each pixel mirror of the DMD array is in the "on" position, the light image will be an array of bright pixels.
For a more detailed discussion of the DMD device, cross-reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method"; U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard Independent Digitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled "Printing System Exposure Module Alignment Method and Apparatus of Manufacture", each patent being assigned to the same assignee of the present invention and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the light image is achieved by pulse width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652 entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System" assigned to the same assignee as the present invention, and the teachings of which are incorporated herein by reference.
In one typical DMD device, each picture element (PIXEL) is typically comprised of a hinge suspended between a pair of support posts, this hinge supporting a 17 micron square mirror above underlying address circuitry. This mirror can be deflected in one of two directions by electrostatic forces generated by underlying electrodes controlled by address circuitry. This pixel is typically fabricated using semiconductor processes including sequentially depositing materials, such as using conventional sputtering techniques, and defining the various structural elements, such as by masking these layers and etching or ashing the exposed (unmasked) material.
The DMD hinge layers ultimately patterned into the hinges may be formed from a variety of metals including, but not limited to, aluminum, aluminum alloys, and titanium tungsten. This hinge metal material is usually deposited by sputtering to achieve a thickness of about 400 to 600 angstroms. Over time, these metal hinges may be become subject to creep because the hinges and beams are preferentially exercised in one direction more than the other, i.e. 75% of the time in one direction and 25% in the other. A hinge/beam exercised equally in both directions will not develop creep. With the current fabrication process, the deposited hinge metal material, such as aluminum alloy, is never exposed to high temperature processing. Therefore, there are few (if any) precipitates in the hinge material. Precipitation normally takes place only during elevated temperature processing. The high temperature process gives the atoms enough thermal energy to diffuse and coalesce at defects or dislocations, forming precipitates and/or intermetallics. The formation of precipitates help pin grain boundaries, reducing the tendency for the hinge material to creep. However, the conventional DMD superstructure, that is, the hinge, beam and support posts, if utilized, do not allow for the use of high temperature annealing exceeding about 400.degree. C. to form precipitates. Therefore, alternative techniques for precipitation formation are desired to minimize or eliminate possible creep.
It is advantageous to reduce the chance for creep in a DMD device, since creep can lead to a preferential set of a DMD pixel mirror in one direction, which degrades device performance including reduced contrast ratios. It is also desirable to provide a robust fabrication process for providing a hardened hinge without the use of high temperature processing.